Oscillator circuits are commonly used in many contemporary circuit applications. Further, as the telecommunications industry advances, higher oscillator frequencies are quite common. For example, under current technology, microwave frequencies on the order of 1 to 300 gigahertz are often used in circuit applications. Typical microwave circuit applications include the use of oscillators for either downward or upward frequency conversion in microwave radio systems. In these microwave applications, as well as for others, it is often very important that the oscillator oscillate only when desired, and that a specific oscillating frequency be maintained during operation. Thus, scientists have invested many years of research and development into building oscillators to meet these goals.
Many different considerations have been examined in order to accomplish the goals set forth above. One key consideration which has been addressed is the bias potential drop across the active device of the oscillating circuit during operation. Specifically, it is beneficial to maintain a constant voltage drop across the active device. This voltage maintenance is important for two reasons. First, a manufacturer of the active device typically provides test data for the device given a specified set of DC criteria. For example, the scattering parameters for the active device are guaranteed by the manufacturer for a stated given DC voltage drop across the active device. Thus, a circuit designer implementing the device typically attempts to match the voltage drop/current flow specified by the manufacturer in order to guarantee these scattering parameters once the active device is operated as part of an oscillator. A second consideration for maintaining a constant voltage drop across an active oscillator device arises from variations in current flow that occur from device-to-device. For example, in a field effect transistor (FET), a known saturation drain-to-source current (I.sub.dss) is expected to flow through the device. Nonetheless, the specific I.sub.dss of an FET is likely to vary from device-to-device. This variation is likely to change the voltage drop across the respective active device. As a result, for oscillators using these devices, oscillation frequency from oscillator-to-oscillator may differ. This result may degrade performance of the overall circuit utilizing the oscillator.
In the prior art, various attempts have been derived in order to individually adjust the voltage drop across the active device of an oscillator. These attempts seek to correct for fluctuations from device-to-device. For example, one technique used in the prior art is to provide an adjustable resistance in series with the active oscillator device and its power supply. The adjustable resistance has been accomplished by way of a potentiometer, or by providing several groups of different values of resistors, any of which may be individually selected and connected between the active device and the power supply. In each instance, the value of the resistance is adjusted or selected to correspondingly adjust the voltage dropped across the resistance. Assuming the supply voltage is constant, this change causes a corresponding change in voltage drop across the active oscillator device in series with the resistance. Accordingly, the voltage drop across the active oscillator device is manually adjustable, thereby allowing bias adjustment and correcting for device-to-device parameter fluctuations.
The manual tuning or selection prior art approaches provide numerous disadvantages. For example, increased time and labor are required for adjusting each oscillator to operate at the necessary or desired voltage bias. Further, these processes quite often require a fairly high degree of technical expertise on the part of the technician. Thus, even with a given level of competence, the manual process is subject to human error. Additionally, device parameter variations may occur after tuning the device. For example, many active devices are known to conduct a different amount of current and, hence, drop a different amount of voltage, when the device is exposed to varying ranges of temperature. As a result, even after a specific oscillator has been hand-tuned, a change of temperature may consequently change the current/voltage biasing the device and, therefore, may alter the oscillation frequency of the device.
In view of the above, it therefore should be appreciated that a need has arisen for an improved circuit and methodology for maintaining the voltage drop across the active device of an oscillator circuit to a given value, thereby minimizing the effects commonly caused by a changing voltage drop.